1. Discovery potential of the Z´ boson with the 2010 ATLAS data Ariadni Antonaki Dimitris Fassouliotis Christine Kourkoumelis University of Athens XXIX Workshop on Recent Advances in Particle Physics and Cosmology 14-16 April 2011, University of Patras, Greece 1

2. “Search for high mass dilepton resonances in pp collisions at sqrt{s} = 7 TeV with the ATLAS experiment” The ATLAS Collaboration arXiv:1103.6218v1 [hep-ex] 31 Mar 2011 This work is part of 2

3. 3 Outline I. Theoretical Introduction II. 2010 ATLAS data III. Monte Carlo Signal and Backgrounds IV. Analysis V. Limits on Z’ ATLAS

4. 4 ATLAS Muon Spectrometer 4 types of Chambers, 2 categories: Precision / Trigger University of Athens (UoA) 30000 BIS MDT tubes assembly National Technical University of Athens (NTUA) Quality Assurance/Quality Control of MDT tubes Aristotle University of Thessaloniki (AUTh) MDT chamber assembly figure from ATLAS Muon Spectrometer TDR

5. Theoretical Introduction Several theoretical models beyond SM predict the existence of new, heavy, gauge bosons at the TeV scale  GUTs  E6 models  Left-Right Symmetric models  Little Higgs models  Kaluza-Klein models Two types of such bosons: Z’ (neutral) / W’ (charged) The present search is based on the Sequential Standard Model (SSM)  Z’ is considered identical to Z (same couplings to fermions) but with much larger mass. The mass is a free parameter in the theory. Limits from Tevatron: ~1TeV We are working with final states to muons: Ζ’  μ + μ - 5

6. 2010 ATLAS data We have used the total amount of 2010 ATLAS p-p data @7TeV, that corresponds to an integrated luminosity of ~42pb -1. 6

7. Signal and Background procedures Signal Z’  μ + μ - (Pythia) @ several mass samples *** Background Procedures with two oppositely charged muons in the final state Sample Drell-Yan to muons (M>60GeV) (high tail of Z  μ + μ - ) Heavy di-bosons production (WW,WZ,ZZ) t\bar{t}  μ + μ - W+jets (a jet misidentified as μ) QCD: b\bar{b}  μ + μ -, c\bar{c}  μ + μ - Cosmic Background (cosmic μ seen as two) 7

8. CUTDescription EVENT SELECTION : Good Run List (GRL)All parts of the detector were operational (exl.probl.LB) TriggerSingle muon triggers were used Primary VertexA vertex with z0<200mm, with at least 3 tracks Bkg rejection MUON SELECTION : TWO combined* muons, oppositely charged, each passing: Dibosons, Wjets, P T > 25GeVReject low-Pt proceduresQCD |η| < 2.4 Trigger acceptance Isolation: ΣP T (cone 0.3) / P T < 5% Jet rejectionQCD / W+jets |d0| < 0.2mm, z0(p.v.) < 200mm, |z0- z0(PV)|<1mm I.P: Cosmic rejectionCosmic Inner Detector HitsPixel, SCT, TRTFakes Muon Spectrometer HitsEnsures good muon resolution Dimuon Mass > 70GeV ANALYSIS: Event & Muon Selection * recon. by ID & MS 8

9. Mass (GeV)70-110110-130130-150150-170170-200 Z/γ*7546.7 ± 7.198.4 ± 3.133.4 ± 1.117.2 ± 0.612.8 ± 0.5 t\bar{t}6.0 ± 0.62.4 ± 0.31.7 ± 0.21.2 ± 0.1 Dibosons10.0 ± 0.50.8 ± 0.10.6 ± 0.00.5 ± 0.00.4 ± 0.0 W+jets0.3 ± 0.20.0 ± 0.0 QCD0.1 ± 0.00.0 ± 0.0 TOTAL7563.0 ± 7.2101.6 ± 3.135.7 ± 1.218.9 ± 0.714.4 ± 0.5 DATA75631014111 Mass (GeV)200-240240-300300-400400-800800-2000 Z/γ*7.8 ± 0.35.1 ± 0.22.5 ± 0.11.3 ± 0.10.1 ± 0.0 t\bar{t}1.0 ± 0.10.7 ± 0.10.4 ± 0.00.1 ± 0.00.0 ± 0.0 Dibosons0.3 ± 0.00.2 ± 0.0 0.1 ± 0.00.0 ± 0.0 W+jets0.0 ± 0.0 QCD0.0 ± 0.0 TOTAL9.1 ± 0.46.0 ± 0.23.0 ± 0.11.5 ± 0.10.1 ± 0.0 DATA76210 ANALYSIS: # of events, after all cuts ( BKG normalized to Data events in Z pole ) Total cosmic background (M>70GeV): 0.004 ± 0.002 9

10. ANALYSIS: Dimuon Masses, after all cuts M μ+μ- = 768GeV 10

11. ANALYSIS: Event @ 768GeV p-value: The probability, in the absence of signal, of observing an excess in a particular mass For 768GeV: ~ 22% (probability 22% to get ONE event in 768GeV, if Z’ DOES NOT exist If p<5%, we wouldn’t be able to put limits) Therefore, NO statistically significant excess above the predictions of the SM is observed. 11 P T 1: 186 GeV / (η,φ) : (-2.39, -1.54) P T 2: 135 GeV / (η,φ) : (0.46, 1.95)

12. 12 ANALYSIS: Limits From BKG pseudoexperiments From DATA 95% C.L. Expected Mass limit (TeV) Expected σΒ limit (pb) Observed Mass limit (TeV) Observed σΒ limit (pb) Z SSM ΄  μμ0.9000.2010.8340.297

13. 13 SUMMARY We have searched for high-mass Z’ boson resonances using the total amount of 2010 ATLAS data. The invariant mass distribution is well-described by the estimated background - We found no excess at high mass in the data. We have set limits on the cross section for Z’ boson production, as well as limits on its mass (0.297 pb - 0.834TeV) Combined limits (using μ+μ- & e+e- channel): Mass : 1.048TeV / σ Β = 0.094pb Limits from CMS: 1.14TeV ( arXiv: 1103.0981v2 )

14. Back Up Slides 14

15. Good Run List (GRL): to ensure that relevant parts of the detector were operational, several Data Quality flags: Muon Detector, Inner Detector, Trigger, Muon Reconstruction // exclusion of problematic Luminosity Blocks. Trigger: Single muon triggers were used // rejection of low-P T physics procedures (different for the several data runs: adjusted to increasing lumi – common for all MC samples) Primary Vertex: The event is required to have a primary vertex in z0<200mm form I.P., with at least three tracks associated to it // ensures a p-p collision 15  Inner Detector Hits: expectBLayer=false OR numberOfBLayerHits >= 1 Pixel Hits + Crossed Dead Pixel Sectors >= 2 SCT Hits + Crossed Dead SCT sensors >= 6 Pixel Holes + SCT Holes <= 1 Restrictions on TRT Outliers (hits close to a track, but not associated to it)  Muon Spectrometer Hits: Inner, Middle, Outer MDT/CSC layers: at least 3 hits in each MS φ hits > 0 BEE, EE, BIS78 vetoed (misaligned chambers) Analysis Cuts

16. 16 Pile Up - Reweighting We use Pile Up Monte Carlo samples (most representative of the bulk of the data). Correlated Observable: Number of Primary Vertices We calculate the number of Primary Vertices in the Z sample and normalize it to the data  EVENT WEIGHT # of P.V. 12345678>=9 weight1.79551.22590.88340.68900.56860.49450.45110.45830.4513 BEFORE reweightingAFTER reweighting

17. QCD Bkg estimation A simulated QCD sample (b \bar{b} & c \bar{c} ), after all selection cuts EXCEPT isolation, is normalized to the data in the region ΣP T (cone 0.3) / P T > 10% (“anti-isolation”) And then used to predict the amount passing the actual cut. 17 Cosmic Bkg estimation Observe the rate and mass distribution of events with: 3 0.3mm The number of events in the final sample is obtained by scaling to the number of those expected to pass the initial cuts.

18. 18 Limits Observed sensitivity (RED line): same as above, but with D k from DATA. Distribution of Z’ events: Median of distribution of several pseudo-exps: BLACK dotted line (“expected”) Ensemble of limits: 68% (green), 95% (yellow) envelope of limits Expected sensitivity: generate Monte Carlo pseudoexperiments using only SM processes, in proportion to their expected rate. We find the 95% CL upper limit for each pseudoexperiment for each fixed possible value of M Z’ : D k : expected events from Bkg template in mass bin N k : ( Z’ + D k events) such as Probability < 5%  Z’ events (for rejection) in mass bin Z’ events to cross-section: A: total acceptance & efficiency N Z,Z’ : 70 < M μμ <110GeV

19. 19 Systematic uncertainties 1). Production model uncertainties to signal and DY background. QCD, E/W k-factors - PDFs. QCD k-factor: 3%, E/W k-factor: 4.5% PDF: 3% @ Z pole, 6% @ 1TeV, 9% @ 1,5TeV (k-factors next page) W+jets cross section: 28% (NNLO), tt: 9.5% (NNLO) Diboson cross section: 5% (NLO) 2). Trigger efficiency (event-based) Scale Factor: 1.002±0.002±0.006 (both μ @ Β) / 1.0009±0.0004±0.0011 (both μ @ EC) 1.002±0.001±0.002 (B + EC) 3). Muon reconstruction efficiency. Mainly due to brems. in Calorimeter: 3% effi loss in 1TeV, grows with mass. Conservative estimation on this uncertainty: 100% of itself. 4) Momentum scale & resolution @ high momenta  Gaussian smearing: δ(q/pT) = S1·g1·(q/pT) + S2·g2 S2 (dominated by MS) = (0.18±0.04)TeV -1 (B&EC) / (0.69±0.20)TeV -1 (CSC) S1 determined from smeared MC fit to the data (for B, EC, CSC)

20. 20 SourceZ’ signalBackground Normalization5% PDFs6% QCD k-factor3% Weak k-factorNA4.5% Efficiency3% Resolution3% TOTAL9.4%10.4% Systematic uncertainties

21. 21 K-factors e/w K-factors